1. Field of Invention
The invention relates to a method for inhibiting the growth of a nickel-copper-tin intermetallic (i.e. (Ni,Cu)3Sn4) layer at the (Cu,Ni)6Sn5/nickel interface of a solder joint. More particularly, the invention relates to a method for inhibiting the growth of a (Ni,Cu)3Sn4 layer at the interface of the (Cu,Ni)6Sn5/nickel by adding palladium into solder joints.
2. Description of Related Art
Soldering is a very common process utilized in modern microelectronic packages, such as assembling IC (integrated circuit) devices to printed circuit boards (PCBs) or those to chip carrier substrates. This is primarily attributed to such process is a very efficient, low cost, and high reliable method in connecting components electrically. During soldering, a solder alloy melts and reacts with the soldering pads (possibly coated with surface finishes) of two opposite components to form a solder joint. These solder joints serve not only to provide signal and electric energy communication among devices of a component but also to achieve a mechanical support among components. The state-of-the-art array-array packages can have solder joints count as high as about five thousands. However, solder joints are realized to be the weakest link for the entire electronic product, especially for the region that the solder alloy contacts with the soldering pad(s), i.e. the solder/pad interface. Failure in any joint (or the solder/pad interface) will make the whole package un-operational. In consequence, to advance the mechanical properties of solder joints becomes one of the most important issues to improve the overall reliability of one electronic product.
When a solder alloy and a soldering pad come in contact, the interfacial reaction between them produces an intermetallic compound (IMC) layer(s) on the contact region (referred to as “the interface of a solder joint” hereinafter). The status of the IMC layer(s) growth plays a predominant role in determining the reliability of a solder joint. If the IMC layer is not formed at the interface of a solder joint, the solderability between the solder and the soldering pad will be very poor, leading to an open circuit easily. Conversely, if the intermetallic layer is too thick, it deteriorates the mechanical characteristic of the interface due to the brittle nature of all the known intermetallics.
According to the related art, the soldering pads are usually treated with surface finish in order to prevent oxidation of a soldering pad and to enhance the bonding reliability with a solder alloy. The Ni-based metallization surface finish, such as electroless nickel and immersion gold (ENIG) plating process, is one of the most common metallization techniques utilized in the related art when conducting surface finish. This is primarily based on the fact that the surface gold layer is an excellent oxidation resistance and the underlying nickel layer (or Ni—P alloy) is a good diffusion barrier that prevents an excessive IMC growth at the interface of a solder joint. By applying the ENIG process, the soldering pads are coated with a bi-layer structure of Au/Ni(P) accordingly.
Additionally, in response to recent enhancement of environmental protection consciousness, lead-free soldering has been extensively used in the microelectronic industry. A tin-silver-copper (Sn—Ag—Cu) family alloy is the mainstream solder alloy adopted to the lead-free soldering process in recent years. The joint microstructure of applying Sn—Ag—Cu alloy to a nickel-containing pad is described hereinafter.
FIG. 1 is a schematic drawing showing the cross-sectional view of a conventional solder joint which is a lead-free, Sn—Ag—Cu solder joint. In FIG. 1, when a solder alloy 110 (made of a Sn—Ag—Cu alloy) and a surface finish 120 (e.g. the Au/Ni bi-layer structure) are soldered, a copper-tin-nickel intermetallic layer (i.e. (Cu,Ni)6Sn5) 130 is formed at the interface of a solder joint. After a long-term use of the solder joints in the normal life, one additional nickel-copper-tin intermetallic layer (i.e. (Ni,Cu)3Sn4) 140 is formed between the copper-tin-nickel intermetallic layer 130 and the surface finish 120, causing the interface embrittlement. The formation of the aforesaid structure would severely deteriorate the mechanical strength of the interface, thus jeopardizing lifespan of the electronic product.